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The layers of the OSI model
The CCNA exam might
ask you to provide at least three reasons that the "industry" uses
layered interconnection models. The basic reason for using a layered
networking approach is that a layered model takes a task, such as data
communications, and breaks it into a series of tasks, activities, or
components, each of which is defined and developed independently.
Examples of layered networking models include the seven-layer OSI model
(which you need to know inside and out) and the Department of Defense
(DOD) five-layer model (which you don't). It isn't important
for you to memorize the ISO/OSI Reference Model's layers; but it's
useful to know that they exist, and that each layer cannot work without
the services provided by the layer below it.
Reasons for a layered model
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Expect to see a
question on the exam that asks you to identify the reasons a layered
model is used in internetworking. Actually, a myriad of reasons
exist for why a layered model is used, but you should memorize these
possible responses:
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Change: When changes are made
to one layer, the impact on the other layers is minimized. If the
model consists of a single, all-encompassing layer, any change affects
the entire model.
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Design: A layered model
defines each layer separately. As long as the interconnections between
layers remain constant, protocol designers can specialize in one area
(layer) without worrying about how any new implementations affect
other layers.
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Learning: The layered approach
reduces a very complex set of topics, activities, and actions into
several smaller, interrelated groupings. This makes learning and
understanding the actions of each layer and the model generally much
easier.
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Troubleshooting: The
protocols, actions, and data contained in each layer of the model
relate only to the purpose of that layer. This enables troubleshooting
efforts to be pinpointed on the layer that carries out the suspected
cause of the problem.
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Standards: Probably the most
important reason for using a layered model is that it establishes a
prescribed guideline for interoperability between the various vendors
developing products that perform different data communications tasks.
Remember, though, that layered models, including the OSI model,
provide only a guideline and framework, not a rigid standard that
manufacturers can use when creating their products.
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Under its official name, the Open
Systems Interconnection Reference Model, or the OSI model, was
developed by the International Organization for Standardization,
which uses the abbreviation of ISO. And, yes, the full acronym of
the OSI is ISO OSI. |
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The OSI model is a
layered model that describes how information moves from an
application program running on one networked computer to an
application program running on another networked computer. In
essence, the OSI model prescribes the steps to be used to transfer
data over a transmission medium from one networked device to
another. The OSI model is a seven-layer model developed around five
specific design principles: |
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Whenever a discrete level of abstraction is required, a new
layer should be created.
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Each layer of the model should carry out a well-defined
function.
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The function of each layer should define internationally
standardized protocols.
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The boundaries of the layers should be placed to minimize the
flow of information across interfaces.
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There should be a sufficient number of layers defined to
prevent unnecessary grouping of functions and the number of layers
should also be small enough so that the model remains manageable.
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